In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. Various combinations of shift type (upshift or downshift) and power flow (power-on or power-off) have been analyzed for control purposes in order to achieve consistent high quality shifting. Power-on upshifting, the subject of this invention, is generally characterized as comprising three phases: a fill phase in which an apply chamber of the on-coming clutch is filled with hydraulic fluid in preparation for torque transmission, a torque phase in which a torque exchange between on-coming and off-going clutches occurs without a corresponding speed change, and an inertia phase in which the input shaft changes speed.
Various techniques have been used for electronically controlling the above-mentioned phases of a power-on upshift; see, for example, the U.S. Pat. No. 5,058,460 to Hibner et al., issued on Oct. 22, 1991, and assigned to the assignee of the present invention. In Hibner et al., the fill, torque and inertia phases are carried out by successively executed control routines. In the fill phase, the on-coming clutch pressure (referred to herein as Ponc) is commanded to a maximum value for a predetermined fill time, or until a pull-down of the input speed is detected. The torque phase is then carried out by releasing the off-going clutch, reducing Ponc to an initial value, and then progressively increasing Ponc until a pull-down of the input speed is detected. Thereafter, the inertia phase is carried out by adjusting Ponc as required to drive the input speed to its post-shift level in accordance with a predetermined speed vs. time profile. In Hibner et al., the clutch pressure (on-coming and off-going) is controlled in an open-loop manner during the fill and torque phases, but is controlled in a closed-loop manner based on input speed error during the inertia phase. In another technique described in the U.S. Pat. No. 4,707,789 to Downs et al., issued on Nov. 17, 1987, and assigned to the assignee of the present invention, the clutch pressures are controlled in an open-loop manner, and the open-loop parameters are adaptively adjusted after a shift is completed based on parameters measured during the shift, such as the inertia phase time or the occurrence of over-filling or under-filling.